Lube-Tech PUBLISHED BY LUBE: THE EUROPEAN LUBRICANTS INDUSTRY MAGAZINE
No.121 page 1
The iodine value as a measure of the stability of ester base oils
Authors: Dr. Herman-Josef Stolz, Dr. Martina Mennicken-Meuthen, Dr. Wilhelm Huber and Franziska Günther; Peter Greven GmbH &
Co.KG
Introduction
The use of synthetic esters as base liquids is due to the fact that these fluids are liquid even at low temperatures. For some applications pour points of less than - 60°C are specified, requirements of values below - 40°C are quite common. These properties can only be achieved through the use of: • saturated short-chain fatty acids (chain length C10)
• branched chain fatty acids • unsaturated fatty acids.
In this case, the use of unsaturated fatty acids, however, is often rejected from the outset by general requirements, such as, for example, an iodine value below 5 mgJ2/100 g. The background behind this requirement is in many cases the assumption that only esters with a low iodine value offer sufficient stability. This statement is a wide generalisation and, in addition, completely neglects the fact that stability implies very different behaviour of the base oils. Thus, it is necessary to differentiate between • thermal stability • oxidation stability • hydrolytic stability.
28 LUBE MAGAZINE NO.150 APRIL 2019
Even if the different forms of stability are naturally partly dependent on each other, there are clear differences. As an example, it can be mentioned at this point, that an ester with outstanding hydrolytic stability does not necessarily have a very good oxidation stability.
In addition, this raises difficulties of assessing the different measurement procedures in terms of their significance for the above-mentioned properties. The object of this article is to differentiate between these aspects of the stability of synthetic esters and to produce a reference to the measurement methods. Of course, there are other considerations on the stability of esters, such as the work or shear stability. This, however, is outside the scope of this article.
Stability of synthetic esters Thermal stability
Most applications of lubricants are in a higher temperature range and this presupposes that the base oils used are stable under the given conditions. In general ester compounds show good thermal stability, comparable with that of mineral oil or in some cases even better. The ester bond itself is significantly more stable than a C-C bond.
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